The present invention relates to a device for heat treatment of an object comprising a susceptor having walls surrounding an inner room adapted to receive the object and Rf-field radiating means arranged outside the walls and adapted to heat the walls and by that said object, and a method according to the preamble of the independent method claim.
xe2x80x9cHeat treatment of an objectxe2x80x9d is to be given a broad sense and includes all types of heat treatment of an object conceivable in such a susceptor, such as epitaxial growth of such an object in the form of a poly- or monocrystalline layer on a substrate received in the susceptor and annealing of a semiconductor wafer for removing damages therein caused by for instance ion implantation of dopants and for electrically activating such dopants then.
The invention is particularly but not in any way exclusively directed to such heat treatment at comparatively high temperatures, such as above 1 400xc2x0 C., and is therefor of particular interest for materials needing such high temperatures for different kind of heat treatments as epitaxial growth and annealing. Such materials are for instance SiC, group III-nitrides and diamond.
A problem in common for such devices already known will now be described for the case of epitaxial growth of a layer by Chemical Vapour Deposition on a substrate received in the susceptor without limiting the invention thereto. Such a known device is schematically illustrated in FIG. 1 and 2. The Rf-field radiating means in such devices are formed by a coil 1 surrounding the susceptor 2 and wound around the susceptor from one end to the other in the longitudinal direction of the susceptor. The walls 3 of the susceptor are made of a material, such as graphite or tantalum, able to couple inductive with the Rf-field for inductive heating of the walls by an alternating magnetic field 4 therethrough.
The density of the magnetic flux through the susceptor walls will be higher in the middle 5 than at the ends 6, 7 of the susceptor as seen in the longitudinal direction thereof. In addition, heat losses from the susceptor will be much higher at the ends than in the middle because of the strong heat radiation at the ends having much larger surfaces for heat exchange with the surrounding gas than the middle section of the susceptor. This will in the case of a flow of precursor gases (indicated by an arrow 23) needed for said Chemical Vapour Deposition through the susceptor result in a considerably higher temperature of the gas flow in the middle of the susceptor than at the ends thereof. The epitaxial growth of most materials is strongly dependent upon a temperature gradient in the longitudinal direction of the susceptor. This means that the area for the growth will be limited to a small area with a lower temperature gradient in the middle of the susceptor. This means that a lower number of wafers may be grown in one and the same growth run than would it be possible to utilise a larger area of the inner room of the susceptor for the growth, which will of course result in remarkably higher costs for producing such wafers. Another disadvantage is that the quality of said wafers grown is affected by the temperature gradient, so that the quality of the objects grown will be lower, in spite of the choice of a small area for said growth, than would the temperature be nearly constant in the growth region. These problems are also there in the case of a heat treatment in the form of an annealing of a semiconductor wafer 8 damaged by ion implantation and they are even more accentuated in that case, since variations in the temperature will then result in different degree of activation of dopants in different regions of a wafer or in different wafers annealed in the same run.
The object of the present invention is to provide a device of the type defined in the introduction, which makes it possible to solve the problem discussed above to a large extent.
This object is according to the invention obtained by providing a device of the type defined in the introduction, in which said means comprises at least two coils with at least one turn each following upon each other in a longitudinal direction of the susceptor and a current source arranged to cause a current to flow in each coil in a direction of rotation around the susceptor walls opposite to that of the other coil for heating said walls.
This means that magnetic fields oppositely directed will be created in the walls of the susceptor. and these fields will counteract each other in the region of the susceptor walls between the coils and close thereto, so that the magnetic flux density will be higher closer to the ends of the susceptor than in said region. This so called anti-polar induction heating means that less heat energy is supplied to a region of the susceptor walls where the heat losses are low and more heat energy may be supplied to regions of the walls where the heat losses to the ambient are higher, so that the temperature gradient in the longitudinal direction of the susceptor may be kept considerably lower than in the known devices described above. In the case of epitaxial growth by Chemical Vapour Deposition this means a possibility to use a larger area of the inner room of the susceptor for the growth resulting in a higher number of objects grown in the same run and by that a saving of costs. Additionally, the quality of the objects grown will also be higher as a consequence of a more uniform temperature inside the susceptor. In the case of using the device for annealing of wafers the much more uniform temperature through the susceptor will make it possible to considerably reduce the occurrence of different degrees of activation of dopants in different regions of a wafer or in different wafers annealed in the same run.
According to a preferred embodiment of the invention each of said coils surrounds substantially one half of the susceptor with respect to said longitudinal direction thereof. This means that the region with a decreased heating will be located in the middle of the susceptor and more heat will be supplied in the directions of the end of the susceptor, so that an area of a substantially constant temperature inside the susceptor may be extended remarkably in the longitudinal direction of the susceptor.
According to another preferred embodiment of the invention at least one of the coils has a plurality of turns, and the heating of the susceptor may of course be controlled better with a plurality of turns of the coils than would each coil only have one turn, although such an embodiment is also within the scope of the present invention and conceivable for very short susceptors.
According to another preferred embodiment of the invention said coils have different numbers of turns, which they may have for different reasons, such as a wish to have a somewhat lower temperature inside the susceptor at one end thereof. Another case with different numbers of turns in the coils is a device in which a first end of the susceptor as seen in the longitudinal direction thereof has thicker walls than the other, and the coil located closest to said first end will then have more turns than the other coil.
According to another preferred embodiment of the invention said coils are formed by one and the same conductor by changing the winding direction thereof, so that one and the same current from the current source will flow through the coils. This constitutes a simple and cost efficient way to realise more than one coil, since only one current source is needed and no control equipment is needed for synchronising a control of separate current sources for separate coils. However, it is pointed out that it may in some cases be preferred to have coils made of separate conductors for obtaining currents of different intensities in the two coils, in which the currents may have a fixed relation to each other or means may be arranged for individually controlling the current intensity of each coil. This means that more heat may be supplied to a region of the susceptor walls where the heat dissipation by radiation is higher and a high temperature may be accepted.
According to another preferred embodiment of the invention the susceptor and heating means are adapted to carry out a heat treatment of an object of SiC, one or more group III-nitrides, an alloy of SiC and one or more group III-nitrides, or diamond. These materials are all of the so called xe2x80x9cwide band gap typexe2x80x9d, which means that the energy gap between the valence band and the conduction band is wide, and comparatively high temperatures are needed for heat treatment, in the form of epitaxial growth or annealing, of these materials. In the case of SiC, the epitaxial growth is preferably carried out at a temperature above 1400xc2x0 C., preferably above 1500xc2x0 C., and very preferred between 1500 and 1700xc2x0 C., but a possibility to grow at a high growth rate will be even higher at a temperature between 1700 and 2100xc2x0 C. A temperature in this region is also needed for the annealing process of an object of SiC.
A method for heat treatment of an object in a susceptor according to the appended independent method claim is also a part of the present invention.
The advantages of such a method and the preferred embodiments thereof defined in the dependent method claims will appear clearly from the above discussion of the device according to the present invention.
Further advantages and advantageous features of the invention appear from the following description and the other dependent claims.